Measles is increasing as a cause of morbidity and mortality worldwide. Measles virus (MeV), the causative agent of measles, is spread by the respiratory route and is one of the most highly infectious viruses of humans with an estimated basic reproductive rate (R0) of 12-18 that drives the need for high levels of population immunity to prevent outbreaks. Measles is a human disease, but macaques can contract measles through contact with humans, develop disease that mimics human measles and have been effectively used for studies of measles pathogenesis. Understanding the extremely efficient airborne transmission of MeV requires knowledge of both the initiation of infection after viral aerosol or droplet contact with the respiratory tract and mechanisms of virus control and release. Respiratory epithelial cells were long assumed to be the first site of MeV infection with subsequent spread to lymphoid tissue. However, investigators using enhanced green fluorescent protein (eGFP)-expressing recombinant reporter MeVs were not able to detect infected respiratory epithelial cells early after infection of macaques or to demonstrate infection from the apical surface of differentiated respiratory epithelial cells in culture and deduced that they had not become infected. Because eGFP-expressing cells were observed after basal infection of cultured epithelial cells, an alternate view emerged that lung epithelial cell infection is initiated through basolateral exposure to infected lymphocytes and does not occur until after the viremia is established. These latter studies have concluded that myeloid cells rather than epithelial cells are the initial sites of MeV infection in the lung. However, our data show that apical infection of respiratory epithelial cells is actually quite efficient, but exposure to MeV induces shedding of MeV- producing multinucleated giant cells from the epithelial surface and leaves it without detectable infected cells. Therefore, epithelial cells may be important for the initiation as well as dissemination of MeV infection. Identification of the susceptible cells in the respiratory tract that allow for very efficient initiation of infection and determination of the mechanism of MeV entry into differentiated epithelial cells are key to understanding efficient MeV transmission and use aerosolized vaccine for measles immunization. To address this critical area we will use in vitro and in vivo studies of rhesus macaques to identify the mechanisms by which both wild type and vaccine strains of MeV infect primary differentiated respiratory tract epithelial cells through the following specific aims: (1) Determine the relative susceptibility to infection with MeV of primary alveolar macrophages and cultures of differentiated cells from the upper and lower respiratory tract that include lung fibroblasts, dendritic cells and basal, ciliated and mucous-producing epithelial cells. (2) Determine the host receptors used and mechanisms by which MeV infects and induces shedding of primary differentiated respiratory epithelial cells after interaction at the apical surface. (3) Identify the innate responses of differentiated respiratory epithelial cells to MeV infection and their role(s) in restricting virus replication.